What is the likely cause and treatment for pain at the mid lateral base of the deltoid in an adult with a history of physical activity?

Mid-Lateral Deltoid Pain: Likely Diagnosis and Management

The most likely cause of pain at the mid-lateral base of the deltoid in an active adult is deltoid muscle or tendon injury, often associated with underlying chronic rotator cuff pathology, and should be managed with relative rest, eccentric strengthening exercises, and NSAIDs for short-term pain relief, with MRI reserved for persistent symptoms or diagnostic uncertainty. 1, 2, 3

Primary Differential Diagnosis

Deltoid Muscle/Tendon Injury

• Deltoid tears occur in 0.3% of shoulder MRI examinations and are strongly associated with chronic massive rotator cuff tears (present in 100% of deltoid tear cases) 3
• The mid-lateral deltoid region corresponds to the middle (acromial) portion, which acts primarily as a shoulder abductor and humeral head stabilizer 2
• Tears typically occur at the musculotendinous junction (79% of cases) rather than at the acromial origin 3
• Partial thickness tears tend to involve the undersurface of the deltoid muscle and tendon 3

Associated Rotator Cuff Pathology

• All patients with deltoid tears have concurrent full-thickness rotator cuff tears, with 92% showing associated muscle atrophy 3
• Chronic rotator cuff tears lead to superior humeral head migration, causing repeated impingement and secondary deltoid injury 2, 3
• Subcutaneous edema overlying the deltoid is present in 63% of cases with deltoid tears 3

Clinical Evaluation

Key History Elements

• Insidious onset of load-related localized pain coinciding with increased activity or new activity 1
• Pain initially present during activity but may subside after warm-up, progressing to rest pain in later stages 1
• Pain described as "sharp" or "stabbing" 1
• History of overhead activities or repetitive shoulder use 1, 2

Physical Examination Findings

• Well-localized tenderness to palpation at the mid-lateral deltoid base 1
• Muscle atrophy (indicates chronicity) 1
• Swelling, erythema, or asymmetry commonly noted 1
• Pain reproduction with resisted shoulder abduction 1
• Limited range of motion on the symptomatic side 1
• Palpable defect in severe cases (8% of deltoid tears) 3

Rotator Cuff Assessment

• Hawkins' test: 92% sensitive for supraspinatus impingement (forcible internal rotation with arm at 90° forward flexion) 1
• Neer's test: 88% sensitive (pain with full forward flexion between 70-120°) 1

Imaging Strategy

Initial Imaging

• Plain radiography should be obtained first to exclude bony abnormalities, loose bodies, or osteoarthritis, though it cannot demonstrate soft-tissue changes of tendinopathy 1

Advanced Imaging Indications

Reserve for: 1

• Diagnosis remains unclear after thorough history and physical examination
• Recalcitrant pain despite adequate conservative management (3-6 months)
• Preoperative evaluation

MRI Without Contrast (Preferred)

• MRI is the reference standard for soft-tissue injuries, showing deltoid tears, rotator cuff pathology, and associated findings 1
• Demonstrates intramuscular cysts, subcutaneous edema, and muscle atrophy associated with deltoid tears 3
• 78% sensitive and 86% specific for tendinopathy 4

Ultrasonography (Alternative)

• 94% specific for tendinopathy, showing tendon thickening, decreased echogenicity, and calcification 4
• Allows dynamic imaging and focused evaluation 1
• Less expensive than MRI but operator-dependent 1

Treatment Algorithm

First-Line Conservative Management (3-6 months trial)

Relative Rest and Activity Modification

• Reduce activity to decrease repetitive loading but avoid complete immobilization to prevent deconditioning 1, 4
• Continue activities that don't worsen symptoms to prevent muscular atrophy 4

Eccentric Strengthening Exercises

• Gold standard treatment achieving full recovery in approximately 80% of patients within 3-6 months 4
• Stimulates collagen production and guides normal alignment of newly formed collagen fibers 1, 4
• Most effective treatment for reversing degenerative changes 1

Pain Management

• NSAIDs (oral or topical) for short-term pain relief, though they don't alter long-term outcomes 1, 4
• Topical NSAIDs preferred to eliminate gastrointestinal hemorrhage risk 4
• Cryotherapy (ice through wet towel for 10-minute periods) for short-term pain relief 4

Corticosteroid Injections (Consider for Acute Phase)

• More effective than oral NSAIDs for acute-phase pain relief but do not alter long-term outcomes 1
• Should not be used as sole treatment 1

Second-Line Options for Refractory Cases

Extracorporeal Shock Wave Therapy

• Safe, noninvasive, effective but expensive means of pain relief for chronic tendinopathies 1

Surgical Referral

• Effective option for carefully selected patients who have failed 3-6 months of conservative therapy 1, 5
• Techniques include excision of abnormal tendon tissue and longitudinal tenotomies to release scarring and fibrosis 1, 5

Critical Pitfalls to Avoid

• Do not overlook underlying rotator cuff pathology—all deltoid tears are associated with rotator cuff tears 3
• Presence of subcutaneous edema or intramuscular fluid signal should raise suspicion for deltoid tear 3
• Do not rely on corticosteroid injections alone—they provide only short-term benefit without changing long-term outcomes 1
• Avoid complete immobilization, which causes deconditioning and muscle atrophy 4
• Do not proceed to surgery without adequate 3-6 month conservative trial 1, 5